The field of the invention is synthetic resins comprising a rubber and an aromatic resin and the invention is particularly concerned with the production of impact-resistant styrene polymers by polymerizing the styrene in the presence of rubber.
Among the prior art methods, continuously implemented bulk and solution polymerizations have become industrially significant.
The state of the art of polymerizing styrene in the presence of rubber may be ascertained by reference to U.S. Pat. Nos. 3,395,746; 3,428,712; 3,511,895; 3,513,145; 4,042,768; 4,081,497; 4,144,204; 4,153,501 and 4,185,049; British Pat. Nos. 1,213,467; 1,547,048; 1,555,725 and 1,576,722; West German published applications 2,017,277 and 2,630,929, the disclosures of which are incorporated herein.
The polymerization takes place in several stages, where in the first polymerization state or prepolymerization stage, the styrene rubber solution, possibly also containing an inert diluent, is polymerized with shearing applied until the polymer is converted beyond phase reversal and then polymerization is continued to completion to the desired styrene conversion with less shear or without any shear at all. The methods conventional in the prior art differ with respect to the stages following the first stage, essentially in that the last stage is carried out in flow reactors, for instance tubular reactors, with plug flow features as disclosed in U.S. Pat. Nos. 2,694,692; 3,243,481; and 4,144,204; British Pat. Nos. 1,155,725 and 1,576,722; and German published application 2,630,929. The other method is a continuous stirred tank reactor cascade consisting of at least three agitation vessels as disclosed in U.S. Pat. No. 3,511,895.
The products obtained by these different prior art polymerization methods differ in the structure of the gel phase, as shown by electron-microscopic examination.
It is furthermore known that the properties of the final products essentially are codetermined by the particle sizes of the soft component portion (particle size and size distribution of the dispersed rubber phase) and by the gel portion of the soft component consisting of the grafted rubber particles and the enclosed polystyrene. The gel portion depends on the kind and the amount of rubber, the polymerization temperature, the agitation conditions applied and the kind and concentration of any added initiator. It is known furthermore that the degree of swelling of the soft component must be accounted for, since this is a measure of the crosslinking density of the gel phase. Lastly the mechanical and thermal properties of the hard component (polystyrene) must be considered. These mechanical and thermal properties are determined by the molecular weight and the molecular weight distribution. In this respect, applicants refer to the comprehensive publications by Willersinn, Makromolekulare Chemie 101 (1967), pp. 296-319; Fischer, Die Angewandte Makromolekulare Chemie 33, (1973) pp. 35-74; Applied Polymer Symposium 15 (1970) p. 74(d); Freeguard, Brit. Polymer J. 6, (1974) pp. 205-228; Stein, Angewandte Makromolekulare Chemie 38 (1974), p. 67, and furthermore British Pat. No. 1,175,262. It is known too that these conditions depend on how the process is being carried out, i.e., they are process specific.
All bulk polymerization methods require removing a substantial amount of generated heat within a short time if high space-time-yields are to be achieved. The space-time yields in these polymerizations can be increased among other ways by adding organic peroxide initiators. When initiators are used in these continuous methods, the polymerization must be monitored, and care must be taken that on account of too slow a dissociation, fairly substantial amounts of unused initiator can collect and this collection of initiator will spontaneously decompose when there is a rise in temperature due for instance, to inadequate heat removal. Therefore, initiators frequently are undesired in continuous bulk methods, as disclosed in British Pat. No. 1,547,048 and U.S. Pat. No. 4,042,768.
An impact-resistant modified polystyrene can be produced at lower technical requirements and costs than in the flow reactors cited above with plug flow features, using to that end a cascade of several continuous stirred tank reactors that the heat removal and hence the control of the polymerization temperature do not take place through complex means installed in the reactors that would require substantial control costs and in the case of a malfunction could only be repaired with great difficulty.
On the other hand, the state of the art teaches that the use of cascades of continuous stirred tank reactors results in lesser product quality as disclosed in German published application 2,017,277 and U.S. Pat. No. 4,081,497.